The present invention relates to ladder frame assemblies for motor vehicles and methods of forming the same.
In the field of motor vehicles, especially off road vehicles, pick-up trucks and similar vehicles, it has been known to provide a vehicle ladder frame assembly for mounting various components of the vehicle, such as the body, engine, suspension system, etc. Typically, the frame assembly includes a pair of lengthwise parallel siderails generally extending along opposite sides of the vehicle, and a plurality of transverse cross members interconnecting the siderails at spaced positions therealong. The frame assembly acts as the main load carrying member for rigidity and strength, as opposed to the vehicle body members of the "uni-body" construction that are typically used in standard passenger cars.
In more recent years, vehicle frame assemblies have been manufactured with added modularity. More specifically, the vehicle frame assemblies may be conveniently divided into a forward frame module, central frame module, and a rearward frame module. This facilitates manufacture and permits certain frame modules to be used with more than one type or model vehicle. Moreover, modular design has the advantage that different portions of the frame assembly can be manufactured with different constructions. For example the forward frame module is preferably manufactured with closed bar siderails (i.e., having a closed transverse cross-sectional configuration) to provide the frame assembly with maximized strength for crashworthiness and torsional rigidity under the cab. The central and rear frame modules, on the other hand, are preferably manufactured with siderails having a U-shaped transverse cross sectional configuration, which includes a base portion and a pair of leg portions extending in generally parallel relation from opposite transverse ends of the base portion. The opened portion of the U-shaped configuration faces inwardly towards the center of the vehicle to best accommodate the slight flexture of the frame during vehicle operation and to facilitate mounting of cross members and other components. As used herein, the term U-shaped may be synonymous with a "C-shaped" configuration, with the "C-shaped" terminology referring to a more specific U-shaped configuration in which inwardly extending flanges are provided at the opposite ends of the U-shaped configuration.
In producing vehicle frame assemblies, manufacturers typically employ a stamping technique. Stamping is accomplished by utilizing one or more opposing stamping die members which compress (by impact) a sheet of raw metal, usually steel, therebetween to form frame siderails and cross members of a desired shape (e.g., having the U-shaped cross sectional configuration). During the stamping process, the raw material is sequentially moved and stamped in different stamping die stations until the final shape is achieved. Stamping operations are advantageous in some respects, such as its ability to provide a relatively simple means for forming components with a non-uniform or varying cross-sectional configuration. However, the stamping dies and equipment are relatively expensive and must be replaced on a frequent basis. As a result, it can be appreciated that material, labor and equipment costs associated with manufacturing and assembly of stamped frame members is significant.
Another problem associated with stamping is that a significant amount of raw material is wasted, especially where irregularly shaped frame parts are required. More specifically, where irregular shapes are to be stamped from a sheet of raw metal, there is a significant amount of peripheral material (particularly at the side edges of the sheet) that cannot be used, irrespective of how closely the stamped shapes are nested on the sheet. In one analysis, it was found that up to 30%-40% of a roll of raw material could not be used due to the irregular shape of the stamped part. It can be appreciated that, at high volumes, the amount of material wasted, and costs associated therewith, is significant in the stamping method.
Another problem with stamping exists in the assembly of the stamped members into a frame assembly. This is typically done using a wire weld process that applies significant heat to the parts. During the assembly process, the stamped parts must be clamped into position to control the tendency thereof to spring back slightly towards its original shape due to the memory contained in the metal grains, and to control the stresses that are created during the heating and the forming processes. Without clamping, these stresses might result in significant distortion of the assembly (6-12 mm overall are typical). Thus, the assembly and tooling at high volumes results in high investment, manufacturing costs, and rework costs to obtain a quality assembly.
The patent literature has proposed a procedure in which an entire frame siderail is roll-formed rather than stamped. More specifically, in U.S. Pat. No. 2,127,618 (hereinafter "the '618 patent), there is proposed an apparatus which passes a sheet of raw metal material through a series of cooperative rollers which sequentially bend the sheet into a tubular frame member having a box-shaped transverse cross sectional configuration. Roll forming offers numerous advantages over stamping. For example, capital expenditures required for manufacturing equipment is significantly lower. In addition, because roll forming machines function without the stresses of high impact associated with stamping dies, they require less maintenance and have a longer useful life. Furthermore, because a sheet of material can be continuously passed through the series of rollers, frame siderail production times are faster. Additionally, because a sheet (typically contained in a roll; see the '618 patent) of raw material can be used completely in making a siderail, little or no raw material is wasted. Assembly is minimized as the sheet is formed, during the rolling process, into a complete closed box section. It can also be appreciated, therefore, that manufacturing costs associated with roll forming is significantly less than with stamping.
The benefits of roll forming, however, have heretofore been limited by the process' inability to form siderail frame members of any desired shape. For example, in the most desirable configuration, it is advantageous not only to provide the central siderail frame members with a U-shaped cross sectional configuration as aforesaid, but it is also desirable to provide the central portion of the siderail with bends in a vertical direction, for example, to accommodate axle mountings for the rear wheels. Since, unlike stamping, the roll forming process can form only a straight siderail member (e.g., without any bending in the vertical direction), any vertical bends must be accomplished in a subsequent operation. For example, the aforementioned '618 patent contemplates that after formation, the straight box sectioned siderail can be bent in a bending mill to provide forward and rear kickup. While it is relatively simple to bend box-shaped cross sectional configuration because of its inherent strength, heretofore attempts at bending a roll-formed siderail member having a U-shaped configuration have been commercially unsuccessful. More specifically, such attempts have been ineffective due to the relative weakness of an opened section configuration and its susceptibility to unwanted twisting and distortion.
There has thus been a need to manufacture a frame assembly for an automobile that addresses the concerns noted above. It is therefore an object of the present invention to meet this need. In accordance with the principles of the present invention this objective is achieved by providing a method for manufacturing a ladder frame assembly for a motor vehicle. Two of the steps of the method of the present invention comprise forming a rearward frame module and forming a forward frame module. Another step is forming a central frame module comprising a pair of longitudinal central frame siderails, each of the central frame siderails being formed by 1) moving a sheet of metallic material through a series of cooperating rollers so that opposite sides of the sheet are rollingly engaged by the rollers in a manner which bends the sheet to form a longitudinal member having a generally U-shaped transverse cross section including a base portion and a pair of leg portions extending in generally parallel relation to one another from opposite ends of the base portion, and 2) bending the longitudinal member having the U-shaped cross-section so that i) the base portion is bent generally within its plane, ii) one of the leg portions extending from one end of the base portion is stretched, and iii) an opposite one of the leg portions extending from an opposite end of the base portion is compressed. A final step of the method is rigidly securing the front frame module and the rearward frame module to opposite ends of the central frame module.
The forward frame module comprises a pair of front frame siderails which preferably have a closed cross-sectional configuration. Where closed box frame parts are to be manufactured (e.g., at forward portions of the frame which absorb crash energy), stamping becomes even more problematic since a single integrally formed tubular member cannot be stamped. Instead, where tubular frame parts are required, it is necessary to stamp a plurality of elongated sections which must be longitudinally welded. For example, two facing U-shaped sections may be welded into a box-like sectional configuration. Such a construction is not as strong or dimensionally accurate as one in which an integrally formed tubular member is employed. It can also be appreciated that this stamping and welding process requires even further resources, such as welding material and additional man-hours to complete the weld.
It should be noted that roll forming can be used to manufacture a tubular member for the forward portion of the siderail. For example, to form the box-shaped configuration noted in the aforementioned '618 patent, it is necessary to roll the side edges of the strip material into the squared cross-sectional shape until the side edges of the strip material are brought into abutting relation and then welded. More recently, however, it has been proposed to integrally form a box-like tubular frame member from a tubular blank in what is known as a hydroforming process. See, example, U.S. Pat. Nos. 5,107,693, 5,233,854, 5,333,775, 4,567,743, 5,070,717, 5,239,852 and 5,339,667. While hydroformed frame members are highly advantageous for the forward portions of the siderail frame assembly, heretofore, the related art has not provided a modular frame assembly which employs the particular advantages of utilizing the hydroforming process for the forward frame siderail members and employs the particular advantages of the roll forming and bending processes for forming central frame siderail having a U-shaped cross-sectional configuration members.
In accordance with the principles of the present invention each of the front frame siderails are preferably formed by 1) placing a metallic tubular member into a cavity of a die mold, the die mold having an interior surface defining a shape of the cavity, and 2) providing a fluid interiorly to the metallic tubular member with sufficient pressure so as to expand the tubular member outwardly into engagement with the interior surface of the die mold to substantially conform the tubular member to the shape of the cavity.
The hydroforming procedure for forming the front frame module of the present invention shapes a pair of tubular blanks into the front frame siderails. Fluid pressure is applied to the interior of each blank to expand the blank. As a further advantage in accordance with the present invention, the ends of the blank are pushed inwardly during the hydroforming process to obtain material flow of metal so that the wall thickness of the expanded tube is maintained within .+-.10% of the original blank.
The present invention also achieves its objective by providing a frame assembly for a motor vehicle comprising a rearward frame module having a pair of generally parallel longitudinally extending rearward frame siderails, a forward frame module having a pair of generally parallel tubular forward frame siderails, and a central frame module having a pair of generally parallel longitudinally extending central frame rail members each having a generally U-shaped transverse cross-sectional configuration including a base portion and a pair of generally parallel leg portions extending from opposite transverse ends of the base portion. The U-shaped central frame members are each bent at two longitudinally spaced locations so that at a first of the locations the base portion is bent generally within its plane, one of the leg portions extending from one transverse end of the base portion is substantially stretched, and an opposite one of the leg portions extending from an opposite transverse end of the base portion is substantially compressed, and so that at a second of the locations the base portion is bent generally within its plane, the one of the leg portions extending from one end of the base portion is substantially compressed, and the opposite one of the leg portions extending from the opposite transverse end of the base portion is substantially stretched. The central frame module has respective forward ends of each of the central frame siderails connected with respective rear ends of each of the forward frame siderails, and has respective rearward ends of each of the central frame siderails connected with respective forward ends of the rearward frame siderails.
While in the present invention it is preferable for the central frame module to include longitudinal central siderails each formed into a substantially straight longitudinal member having a U-shaped cross-sectional configuration that is subsequently bent to accommodate axle mounting for rear wheels, different advantageous techniques for bending the central frame siderails are available. More particularly, in the instance in which the longitudinal member having the U-shaped cross-section is bent without making some other accommodation, there is a tendency for the longitudinal member to exhibit a slight spring-back toward its original configuration. As noted previously, such spring-back is caused by the memory retained in the grains of metallic material forming the longitudinal member. In order to accommodate this spring-back, it is necessary to over-compensate during the bending operation so that the longitudinal member having the U-shaped cross-section is bent slightly beyond its desired retention point. The slight spring-back brings the longitudinal member to its approximate desired configuration. It can be appreciated that it is somewhat difficult to approximate with accuracy the amount of overcompensation necessary to provide a part of the desired construction.
It is, therefore, an object of the present invention to address the concerns noted above. In accordance with this object, there is provided a method for manufacturing a frame assembly for a motor vehicle, comprising the steps of forming a rearward frame rail module having a pair of laterally spaced elongated rearward frame siderails extending in the fore-aft direction of the frame-assembly; forming a forward frame rail module having a pair of laterally spaced elongated forward frame siderails extending in the fore-aft direction of the frame assembly; forming a central frame rail module comprising a pair of longitudinal central frame siderails, each of the central frame siderails being constructed by forming a sheet of metallic material into a longitudinal member having a generally U-shaped transverse cross section including a base portion and a pair of leg portions extending in generally parallel relation to one another from opposite transverse ends of the base portion; engaging opposite longitudinal ends of the longitudinal member and pulling the longitudinal ends in opposite directions with respect to the longitudinal extent of the longitudinal member in order to stretch the longitudinal member longitudinally so that the metallic material forming the longitudinal member reaches its yield state; and bending the longitudinal member having the U-shaped cross-section at two longitudinally spaced locations so that 1) at a first of the locations the base portion is bent generally within its plane, one of the leg portions extending from one end of the base portion is stretched, and an opposite one of the leg portions extending from an opposite end of the base portion is compressed, and so that 2) at a second of the locations the base portion is bent generally within its plane, the one of the leg portions extending from one end of the base portion is compressed, and the opposite one of the leg portions extending from an opposite end of the base portion is stretched; and rigidly securing the forward frame rail module and the rearward frame rail module to opposite ends of the central frame rail module by 1) securing a forward end of each of the central frame siderails with a rear end of a respective one of the forward frame siderails, and 2) securing a rearward end of each of the central frame siderails with a front end of a respective one of the rear frame siderails.
It has been found that this methodology not only inhibits spring-back to allow the siderail to be manufactured more easily, but also reduces the amount of metal that is required as a result of the stretching operation, thus providing for a material cost savings.
It is a further object of the invention to provide a method of manufacturing a modular frame assembly which employs the aforementioned stretch bending technique for the central frame siderails, which employs a stamping technology for the rear frame siderails to accommodate for an irregular configuration of the rear frame siderails, and which employs a hydroforming technology for the forward frame siderails to accommodate for a high-strength and economical fabrication of the forward frame siderails; thus providing for an optimized modular frame assembly. In accordance with the object, there is provided a method of manufacturing a frame assembly for a motor vehicle, comprising the steps of (A) forming a rearward frame module having a pair of laterally spaced, longitudinally extending rear frame siderails, each of the rear frame siderails having a generally U-shaped transverse cross section including a rear siderail base portion and a pair of rear siderail leg portions extending in generally parallel relation to one another from opposite transverse ends of the rear siderail base portion, the rear siderail base portion having a greater length between the transverse ends at a forward portion thereof relative to the length between the transverse ends at a rearward portion thereof, the rear frame siderails each being formed by i) placing the sheet of metallic material between cooperative stamping dies, and ii) compressively engaging opposite sides of the sheet with the stamping dies so as to provide the sheet with the U-shaped transverse cross section; B) forming from a sheet of metallic material a central frame module having a pair of laterally spaced, longitudinally extending central frame siderails, each of the central frame siderails having a generally U-shaped transverse cross section including a central siderail base portion and a pair of central siderail leg portions extending in generally parallel relation to one another from opposite transverse ends of the central siderail base portion, each of the central frame siderails being formed by i) forming the sheet of metallic material into a longitudinal member having a generally U-shaped transverse cross section including the base portion and the pair of leg portions extending in generally parallel relation to one another from opposite transverse ends of the base portion, ii) engaging opposite longitudinal ends of the longitudinal member and pulling the longitudinal ends in opposite directions with respect to the longitudinal extent of the longitudinal member in order to stretch the longitudinal member longitudinally so that the metallic material forming the longitudinal member reaches its yield state; and iii) bending the longitudinal member having the U-shaped cross-section at two longitudinally spaced locations so that 1) at a first of the locations a) the base portion is bent generally within its plane, b) one of the leg portions extending from one end of the base portion is stretched, and c) an opposite one of the leg portions extending from an opposite end of the base portion is compressed, and so that 2) at a second of the locations a) the base portion is bent generally within its plane, b) the one of the leg portions extending from one end of the base portion is compressed, and c) the opposite one of the leg portions extending from an opposite end of the base portion is stretched, the longitudinal member thus being bent to conform to the desired configuration of one of the central frame sidrails; C) forming from a metallic tubular member a forward frame module having a pair of laterally spaced, longitudinally extending tubular forward frame siderails, each of the forward frame siderails being formed by: i) placing the metallic tubular member into a cavity of a die mold, the die mold having an interior surface defining a shape of the cavity, ii) providing a fluid interiorly to the metallic tubular member with sufficient pressure so as to expand the tubular member outwardly into peripheral engagement with the interior surface of the die mold to substantially conform the tubular member to the shape of the cavity, iii) applying force to opposite ends of the tubular member so as to cause the ends to be moved generally inwardly towards an intermediate portion of the tubular member while the tubular member is being expanded so as to maintain a wall thickness of the tubular member within a specified range; and D) rigidly securing the rearward frame module and the forward frame module to opposite respective ends of the central frame module by i) securing rearward portions of the forward frame siderails to forward portions of the central frame siderails, and ii) securing the forward portions of the rearward frame siderails, each having the base portion with the relatively greater length between transverse ends, to rearward portions of the central frame siderails.
It is a further object of the present invention to provide a method of manufacturing a frame assembly which employs a stamping technique for forming central frame siderails that are bent within the vertical fore-aft plane to accommodate for the rear wheel axles, which stamping technique is economical in that it is not subject to the aforementioned problems associated with wasted raw material. To achieve this objective, the present invention provides a method of manufacturing a frame assembly for a motor vehicle comprising the steps of: forming a rearward frame module having a pair of laterally spaced elongated rearward frame siderails extending in the fore-aft direction; forming a forward frame module having a pair of laterally spaced elongated forward frame siderails extending in the fore-aft direction of the frame rail assembly; forming a central frame module comprising a pair of longitudinal central frame siderails, each of the central frame siderails being constructed by 1) bending an elongated planar sheet of metallic material transversely within its plane ii) placing the planar sheet of metallic material between cooperative stamping dies after is has been bent within its plane; iii) compressively engaging opposite sides of the planar sheet with the stamping dies so as to form the sheet into a longitudinal member having a generally U-shaped transverse cross section; and rigidly securing the forward frame module and the rearward frame module to opposite ends of the central frame rail module by 1) securing a forward end of each of said central frame siderails with a rear end of a respective one of said forward frame siderails, and 2) securing a rearward end of each of said central frame siderails with a front end of a respective one of said rear frame siderails.
It is a further object of the invention to provide a method of manufacturing a modular frame assembly which employs the aforementioned economic stamping technique for the central frame siderails, which employs a stamping technology for the rear frame siderails to accommodate for an irregular configuration of the rear frame siderails, and which employs a hydroforming technology for the forward frame siderails to accommodate for a high-strength and economical fabrication of the forward frame siderails; thus providing for another optimized frame assembly embodiment. In accordance with this object, there is provided a method of manufacturing a frame assembly for a motor vehicle, comprising the steps of (a) forming from an elongated sheet of metallic material a rearward frame module having a pair of laterally spaced, longitudinally extending rear frame siderails, each of the rear frame siderails having a generally U-shaped transverse cross section including a rear siderail base portion and a pair of rear siderail leg portions extending in generally parallel relation to one another from opposite transverse ends of the rear siderail base portion, the rear siderail base portion having a greater length between the transverse ends at a forward portion thereof relative to the length between the transverse ends at a rearward portion thereof, the rear frame siderails each being formed by i) placing a sheet of metallic material between cooperative stamping dies; ii) compressively engaging opposite sides of the sheet with the stamping dies so as to provide the sheet with the U-shaped transverse cross section; b) forming from a planar sheet of metallic material a central frame module having a pair of laterally spaced, longitudinally extending central frame siderails, each of the central frame siderails having a generally U-shaped transverse cross section including a central siderail base portion and a pair of central siderail leg portions extending in generally parallel relation to one another from opposite transverse ends of the central siderail base portion, each of the central frame siderails being formed by i) bending said elongated planar sheet of metallic material transversely within its plane ii) placing the planar sheet of metallic material between cooperative stamping dies after is has been bent within its plane; iii) compressively engaging opposite sides of the planar sheet with the stamping dies so as to form the sheet into a longitudinal member having a generally U-shaped transverse cross section; and c) forming from a metallic tubular member a forward frame module having a pair of laterally spaced, longitudinally extending tubular forward frame siderails, each of the forward frame siderails being formed by: i) placing the metallic tubular member into a cavity of a die mold, the die mold having an interior surface defining a shape of the cavity, ii) providing a fluid interiorly to the metallic tubular member with sufficient pressure so as to expand the tubular member outwardly into peripheral engagement with the interior surface of the die mold to substantially conform the tubular member to the shape of the cavity, iii) applying force to opposite ends of the tubular member so as to cause the ends to be moved generally inwardly towards an intermediate portion of the tubular member while the tubular member is being expanded so as to maintain a wall thickness of the tubular member within a specified range, and d) rigidly securing the rearward frame module and the forward frame module to opposite respective ends of the central frame module by i) securing rearward portions of the forward frame siderails to forward portions of the central frame siderails, and ii) securing the forward portions of the rearward frame siderails, each having the base portion with the relatively greater length between transverse ends, to rearward portions of the central frame siderails.
These and other objects of the present invention will become more apparent during the course of the following detailed description and appended claims.
This invention may best be understood with reference to the accompanying drawings wherein illustrative embodiments are shown.